1. Field of the Invention
The invention relates to the structure of a light gate. In particular, it relates to an array of optical shutters which are selectively opened and closed by external electrical signals to transmit and block light.
2. Background Art
In the past, a variety of light gates have been used in picture forming devices. These optical gates have optical shutters of PLZT (lead zironate titanate doped with lanthanum: lead lanthanum zirconate titanate) or of liquid crystal.
The light gate of this type has been disclosed, for instance, in Japanese Utility Model Application (OPI) No. 113541/1980 (the term "OPI" as used herein means an "unexamined published application" or unexamined PCT Patent Application No. 502242/1982, published in Japan. In this light gate, a ferro-dielectric ceramic panel of PLZT is arranged between two polarizing plates which are so arranged that the polarization axes thereof are perpendicular to each other. Voltage is applied across comb-shaped common electrodes on one hand located on the panel and signal electrodes on the other hand located between the common electrodes (the comb teeth). An electric field is thus created between the signal and common electrodes. As a result, the PLZT between these electrodes is made birefringent, so that the direction of polarization of light passing through the PLZT is rotated. The rotation of the plane of polarization of incident light is controlled according to whether or not voltage is applied across the electrodes. When the polarization axis of the light incident upon of the polarizing plate on the output side of the light gate coincides with that of the plate, the incident light passes through the light gate, and if not, the incident light is blocked thereby.
A variety of light gates using a liquid crystal have been proposed in the art. One example of the light gate has been disclosed, for instance, in Japanese Patent Application (OPI) No. 98073/1981. In the light gate, a liquid crystal layer is formed between two polarizing plates in the same manner as in the above-described light gate using PLZT. Transmission of light through the light gate is controlled by controlling the application of voltage across two transparent electrodes which are formed on both sides of the liquid crystal layer and subjected to rubbing. The general liquid crystal light gate is of twisted nematic type, and the twisted nematic type gates are further classified into a group of negative type and a group of positive type gates
A variety of methods have been proposed in the art in which the above-described light gate is disposed between a light source and an image forming plate (such as a photo-sensitive material). The optical shutters of the light gate are driven by external electrical signals (which may be referred to as "image data") to form an image on the light gate and the output light of the light source passed through the image is utilized to form the image on the photo-sensitive material.
In the picture forming method disclosed in unexamined PCT Patent Application No. 502242/1982, published in Japan, the above-described PLZT light gate is used to form a continuous tone image on the recording medium.
In the picture forming method disclosed by Japanese Patent Application (OPI) No. 98073/1981, the above-described liquid crystal light gate is interposed between a light source and an "electrofax" sheet, and the light gate is driven with electrical signals to form a picture.
In any one of the above-described light gates, the optical shutters are provided for the respective picture elements (pixels) of a picture, (in such a manner that one optical shutter corresponds to one picture element or pixel) and the optical shutters all have the same configuration. Therefore, it is impossible to form a pixel by using the optical shutters of the light gate in combination, or to provide gradations through the variations of area which are obtained by using the optical shutters individually or in combination.
The picture forming method using the PLZT light gate is advantageous in that the optical shutters are higher in response speed than those of the liquid crystal light gate and more suitable for gradation control. However, in the method, the irregularity of the pixels comprising the light gate array results in the formation of stripe-like patterns in forming a picture on the photo-sensitive material. This difficulty is too serious to form a picture rich in gradation